Originally Posted by Jabba
Send a PM to Gringo- he's got one that he installed his own self... tell him his baby brother sent ya~
I have one of these http://www.aircraftspruce.com/catalo...levelmeter.php
from Aerostitch, no longer avail there but thanks to CGR I now know where to get one for wife's bike. Important thing for accuracy is to attach to a key-switched wire that is not heavily loaded; I checked at battery w/handheld DVM and know mine always reads ~1/2V low, and mentally correct. If you're real fussy you could run a wire direct to the battery thru a relay off the switched wire. Multicolored LED's light up in .5V intervals and are very easy to interpret via peripheral vision when underway; a coupla greens, you're good; upper end of the yellows OK, check back to see if they're dropping; only red lights, time to switch off all the extras, get the revs up and monitor the recovery. Comes with 2 faceplates for vertical or horizontal mounting.
I get mine very wet all the time and it has never shorted as some online reports have warned (I fused the (+) wire in case it ever does). It's epoxy-encased and completely waterproof, but wires attach to two screw terminals at opposite ends of the back outside of the box, so you need to make sure they can't short to metal or a big puddle of water. Not hard, but dab 'em with silicon if you're nervous.
There's a bunch of other alternatives at http://faq.f650.com/FAQs/AftermarketVoltmeterFAQ.htm
including schematic for a home-grown version with LED's; you could conceivably wire the LED's with a ribbon cable to a separate unit that would fit neatly somewhere on the GS's crowded dash (drilling out the dummy switch and punching LED's thru it comes to mind). They show a number of other options that are probably as applicable to the airhead GS as to the F650, with a little creativity.